I would be comfortable tileing over the level brick surface "IF" I knew there hadn't been oil or such applied on brick. I think brick might have more tooth than backerboard,therefore making it a non-issue if mortor lines aren't filled. I would use knotched trowel and finish as usual. Doing as I suggest (lazy man's way) will require paying close attention to grouting due to extra voids.

It would help to see the fireplace but i would use larger tiles if possible.

Thanks, I like the skimcoat idea, that should easily give me a nice flat reference surface. The tiles aren't a variable, SWMBO found nice handmade tiles in a perfect A&C style mottled green glaze at a bargain price. They look like the ones on a lot of 1910-1920 fireplaces we've seen, the smaller tiles were common then. They look like the real thing, so we're going with that.

Your sweetie's name is SWMBO ? So is mine,isn't it a small world? I'm no tile man so I check tile level as I go along with 2' leg of frameing square edge for small tile and 4' steel ruler for large tile. I set a few then lay a 1"x2"x2'or4' hardwood board on tile then push and/or hammer board. I expect you already know this but use spacers unless tile have them moulded in. I've seen pros set tile perfectly without spacers but found that watching and doing result in intirly different results for me. I rarly use additives in thin set or grout indoors. Reason being that smears/mistakes cure faster and are more difficult to clean up. If fireplace would burn wood I "might"use additive in grout to resist stain. Are you changing mantle or other features? Best of luck and I trust you will post a pic or two when it's finished. Oh,btw let us know what favors you gain with her. (:

Afraid you'll need to be patient on the "finished" pictures, I have a day job that interferes with getting fun stuff like this done. The old faux colonial mantle is long gone so that the carpenters, insulaters and plasterers could get to the wall behind and fix the holes around the fireplace that were big enough to let small birds fly freely into the house. The new mantle will be cherry, arts & crafts style, probably with inset panels of birdseye maple to match the rest of the woodwork I've been doing in the house. Here's a poor photo of the window trim, we've generally kept it pretty simple:

I may yet break down and hire a pro to do the tile, it's not going to be as easy as regular 4x4 machine made porcelain bathroom tile. No way I can do vertical surfaces without spacers.

Turning it over to pros doesn't make you a bad boy,goodness know's we've all wished we had more than once.